This research will study people's ability to think deductively and its modification in the middle and later years by (a) psychobiological processes of aging and (b) training in logical reasoning. Do biological processes of normal aging, accompanied by limitations on capacity of working memory, permit irrational factors the to interfere with the older person's ability to reason correctly? Preliminary work shows that people's beliefs or factual knowledge affect the way they reason on syllogistic reasoning problems that require an objective, detached approach. Regardless of the logical validity of an argument, both young and old tend to accept congenial conclusions and reject those they disbelieve. This tendency has recently been shown to be markedly stronger in the elderly. Two series of studies are outlined with the shared long term objective of relating cognitive assessment with cognitive training. Both series address the issue of plasticity and variability in old and young college graduates. The first focus on sources of difficulty that cause errors in logical reasoning: belief bias, reduced capacity of working memory, and inability to access abstract knowledge rules that can be mapped on to unfamiliar problems. the second series focus on strategies that people use to reason correctly. We will conduct think-aloud studies to discover what strategies older people use effectively and what sorts of problems best trigger their use. Training studies are planned to apply interventions that have proved successful with college students to teach reasoning.